libgo/go/math/ldexp.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package math
   6
   7 // Ldexp is the inverse of Frexp.
   8 // It returns frac × 2**exp.
   9 //
  10 // Special cases are:
  11 //      Ldexp(±0, exp) = ±0
  12 //      Ldexp(±Inf, exp) = ±Inf
  13 //      Ldexp(NaN, exp) = NaN
  14
  15 //extern ldexp
  16 func libc_ldexp(float64, int) float64
  17
  18 func Ldexp(frac float64, exp int) float64 {
  19         r := libc_ldexp(frac, exp)
  20         return r
  21 }
  22
  23 func ldexp(frac float64, exp int) float64 {
  24         // special cases
  25         switch {
  26         case frac == 0:
  27                 return frac // correctly return -0
  28         case IsInf(frac, 0) || IsNaN(frac):
  29                 return frac
  30         }
  31         frac, e := normalize(frac)
  32         exp += e
  33         x := Float64bits(frac)
  34         exp += int(x>>shift)&mask - bias
  35         if exp < -1074 {
  36                 return Copysign(0, frac) // underflow
  37         }
  38         if exp > 1023 { // overflow
  39                 if frac < 0 {
  40                         return Inf(-1)
  41                 }
  42                 return Inf(1)
  43         }
  44         var m float64 = 1
  45         if exp < -1022 { // denormal
  46                 exp += 52
  47                 m = 1.0 / (1 << 52) // 2**-52
  48         }
  49         x &^= mask << shift
  50         x |= uint64(exp+bias) << shift
  51         return m * Float64frombits(x)
  52 }